uh, i dont know all about it but i believe it is just a function to send certain messages to a window like this: SendMessage(WM_CLOSE,0,0); This for instance would send a WM_CLOSE message to your window and if that window has a handler for the message to close it, the window will be closed. There's all sorts of messages (see winuser.h). WM means its a window message, you also have messages for buttons and other objects, for instance, button messages start with "BN". The two 0's are the wParam and the lParam, these are optional parameters, for example if a window receives a WM_COMMAND message it means someone pressed a button or did another command, then you can find what command by looking at the wParam. Kuniva